Laser etching is a known method to remove a layer of material deposited at the surface of another material. By irradiating the layer of material to be removed, the electromagnetic energy heats up the surface thereof, after which the heat propagates all the way to the interface between the two materials where it is stored until a blister forms. The layer of material to be removed thus separates from the material supporting it.
The energy necessary for the separation characterized by the fluence received from the laser, the irradiation time, and the laser wavelength, or equivalently by the surface power, the irradiation time, and the laser wavelength, depends on the characteristics of the layer of material to be removed and on the characteristics of the material having this layer deposited thereon. The properties of the laser thus have to be adapted to each specific case.
Now, it is frequent for a layer of material to be removed to be deposited on two different materials. For example, on manufacturing of an organic transistor, the metal drain and source electrodes, usually made of gold, are deposited on a plastic substrate, after which an organic semiconductor layer having a thickness of some hundred nanometers is deposited and covers the assembly. At this stage of the manufacturing, the electrodes then need to be exposed. However, irradiating the semiconductor layer with a fluence and an irradiation time selected to separate the portion of semiconductor layer deposited on the plastic substrate also deteriorates the metal electrodes, or even separates them from the plastic substrate.
Thus, for example, to separate a 100-nanometer semiconductor layer deposited on a polyethylene naphthalate (PEN) substrate, a minimum fluence of 70 mJ/cm2 is required for an irradiation of 30 nanoseconds with a laser at 245 nanometers. Now, such an irradiation is incompatible with gold drain and source electrodes deposited on the substrate since these electrodes are deteriorated as soon as the fluence is greater than 55 mJ/cm2. It is thus impossible to separate both the portion of semiconductor layer deposited on the plastic substrate and the portion of semiconductor layer deposited on the metal electrodes by means of one and the same irradiation applied to the entire organic semiconductor layer. Usually, this layer is thus removed by means of a chemical processing, which has the disadvantage of leaving residues.